In the field of subsea oil and gas production it is known to arrange a subsea Xmas tree (XT) on the seabed, on top of a subsea well extending into the seabed. The XT controls the flow of fluids, in particular produced hydrocarbons flowing out from the well. When used on an injection well, it controls fluid injected into the well. Moreover it constitutes a barrier between the well and the environment. It is known to connect the XT to a wellhead or a tubing head spool at the top of the well. For conventional Xmas trees, also referred to as vertical Xmas trees, a tubing hanger (TH) is hung off in the wellhead or the tubing head spool below the XT. The tubing head spool is an adapter unit in between the wellhead and the vertical XT usually featuring an annulus isolation valve.
The type of XT which will be discussed herein is the type known as a vertical Xmas tree having two bores. One bore is a main bore, known as a production or injection bore. The second bore is normally referred to as the annulus bore which communicates with the tubing annulus. The tubing annulus is the annular space between the production tubing and the well casing. The tubing annulus is also referred to as the annulus A. There is also an annulus B which is the interspace between the inner casing (outside the tubing) and the next casing. The annulus bore in the XT communicates with an annulus in the wellhead, through a bore extending vertically through the TH and through a part of the Xmas tree main body. A vertical (conventional) dual bore XT has access to the TH main bore (production/injection bore) and the TH annulus bore, through dual bores that extend through the XT main body. These bores are accessible from topside to allow plugs to be set by wireline through a workover riser, e.g.
Normally the wellhead dimensions are considered as an industry standard which is widely spread. The dimensions thus restrict the possibility to redesign the wellheads to allow larger production/injection bores through the stack of wellhead, TH, and XT.
The limited space available here limits the possible space for a vertical annulus path to enable wireline operations from topside in the TH through Xmas tree.
After running the completion of the TH and the tubing, the wellhead or the tubing head spool is temporarily isolated until the XT is landed and connected. This means that the production bore and the annulus bore of the TH are isolated by running a plug in the respective bores. This is a conventional method for isolation of an abandoned well.
The dual bore design of a vertical XT allows access for a wireline tool to set and unset a plug in a plug-profile in the annulus bore in TH. However, in the case of a XT with a deviated annulus bore, the wireline tool cannot access the TH annulus bore. In such cases the technology of prior art uses a valve in the TH or in a tubing head spool to constitute isolation of the annulus.
US patent publication U.S. Pat. No. 7,121,344 discloses a plug tool which is adapted to land on top of a XT, and extend a stem through the XT in order to set or retrieve a plug in the bore of the TH.